Step stool



FIG. 1 is a perspective view of the step stool;

FIG. 2 is a front view of the step stool;

FIG. 3 is a side view of the step stool;

FIG. 4 is a top view of the step stool; and,

FIG. 5 is a bottom view of the step stool.

The portions of the step stool shown in broken lines form no part of the claimed design. 

CLAIM The ornamental design for a step stool, as shown and described. 